As the technique for a production of printed circuit boards, there has widely been known so-called additive process of forming a conductor pattern on a substrate through electroless copper plating treatment. In this process, there is generally adopted a method, in which a catalyst nucleus is applied to an insulating substrate and an electroless plated thin film (primary plating) is directly formed thereon and then an electroless plated thick film (secondary plating) is formed on the plated metal surface, as the electroless plating treatment for the formation of the conductor pattern.
In such an electroless plating treatment, the primary plated film first deposits around the catalyst nucleus applied onto the surface of the insulating substrate and the deposition of the secondary plated film proceeds on the metal constituting the primary plated film after the formation of the primary plated film layer. Therefore, different properties are required in the primary plating and the secondary plating.
That is, when the plated film is deposited around the catalyst nucleus formed on the surface of the insulating substrate as in the primary plating, it is required that the deposited particles are fine and the thickness of the plated film is uniform.
When the plated film is deposited on the metal constituting the primary plated film as a nucleus as in the secondary plating, it is required to accelerate the deposition rate to increase productivity.
Particularly, the usual electroless plating treatment uses a plating solution containing ethylenediamine tetraacetate (hereinafter abbreviated as EDTA) as a complexing agent and which is slow in the deposition rate, so that it is necessary to accelerate the deposition rate of the secondary plating.
On the other hand, JP-A-1-168871 and JP-A-2-30770 have proposed an electroless plating solution containing triethanol amine (hereinafter abbreviated as TEA) as a completing agent as a means for accelerating the deposition rate.
However, the electroless plating solution containing TEA as the completing agent can be used in the primary plating to be conducted after the application of catalyst nucleus such as Pd or the like without problems, but when such a solution is used in the secondary plating for depositing the metal constituting the primary plated film as a nucleus, the initial plating deposition is very poor and there are problems such as lack of deposition, reaction termination and the like.
In order to solve the problem in the secondary plating, there has been proposed a method of improving the bath reactivity by increasing a concentration of a reducing agent such as formaldehyde (HCHO) or the like added to the electroless plating solution containing TEA as a complexing agent (not less than 3 cc/l (0.037 mol/l)).
In this method, however, the reducing agent strongly reduces copper ions in the plating solution to cause the deposition of unnecessary metallic copper, so that there are troubles such as occurrence of nodule and decomposition of the plating solution.
Furthermore, there is a method in which a catalyst nucleus such as Pd or the like is applied onto the plated film formed by the primary plating and subjected to the secondary plating as another means for solving the problem in the secondary plating.
In this method, however, the cost undesirably increases, and also it is difficult to apply and fix the Pd catalyst nucleus to the metal film different from the insulating substrate and hence there is a problem of abnormal deposition due to drop-off of Pd catalyst nucleus.
On the contrary, there have hitherto been proposed the following pretreating solutions for electroless plating:
1 a pretreating solution having a chemical copper plating composition other than copper complexing agent and copper compound (JP-A-63-129692);
2 a pretreating solution for electroless plating consisting of alkali hydroxide, metal complexing agent and alkali metal halogenide (JP-A-2-30768); and
3 a pretreating solution for electroless plating consisting of acid, metal complexing agent and reducing agent (JP-A-2-30769).
However, these pretreating solutions are solutions for depositing the plated film onto the insulating surface, but are not solutions for depositing the secondary plated film onto the surface of the primary plated film and hence they do not accelerate the initial deposition rate in the secondary plating.
As mentioned above, it is difficult to improve the initial deposition of the secondary plating without any troubles in the above conventional techniques, so that there is no practical method of accelerating the deposition rate of the secondary plating.
It is an object of the invention to solve the aforementioned problems of the conventional techniques and to provide a technique for electroless plating treatment capable of improving the initial deposition of the secondary plating without any drawbacks.
It is another object of the invention to accelerate the plating deposition rate in the secondary plating.